OK; I’ve got a bunch of 1.5V DC grain of wheat bulbs, and a bunch of 12V DC power sources. Could I use the same resistors as I do with 1.5V LEDs running off 12V DC sources? Or will something fry?
Just askin’…
Yep, resistors work to dim and protect bulbs, too.
There is a difference, though. Even a small bulb draws far more current than a LED. The small 1/4 watt resistors typiclaly used with LEDs may have to be bumped up to the larger 1/2 watt variety. With 1.5 V bulbs, that’s usually not an issue. However, it’s still good to closely monitor a bulb and resistor install until you’re sure heat won’t be an issue. Melted motrive power is not cool.
Caution technical content
The resistor is used to drop the voltage applied to the bulb from 12v to 1.5v ie it drops about 10.5volts. The amount of current flowing is determined by the value of the resistor and the bulbs resistance when it is lit/hot (When it is cold the resistance is much lower thats why bulbs often blow when you first turn them on).
If you use just a 1.5 volt battery and an ammeter to measure the current flowing when the bulb is lit (hot) you can work out it’s hot resistance R=V/I. (R= resistance in Ohms, V = voltage in Volts, I= current in Amps) So if it was 100milli Amps then R= 1.5 /0.100 = 15 Ohms
Now add the value of your resistor and the resistance of the hot bulb together. Say 1000 +15 =1015Ohms.
Work out the current that would flow if you connected it to 12Volts. I = V/R ie 12/ 1015 = 0.011 Amps.
Now you won’t blow the bulb, because the current is now only 11milliAmps but the bulb needs 100 to light up and I suspect that at 11mA it will not light up.
If you put in your values and end up with a current greater than the 1.5Volt battery uses to light the bulb, the chances are you will blow your 1.5 volt bulbs so increase the value of the resistor.
If you’re prepared to sacrifice a bulb or two, just try different value resistors, but doing the math will save you the anguish of blowing bulbs.
LEDs work differently and tend to drop a constant voltage.
Hope I haven’t confused you.
A simple calculation is applied Voltage (12 Volts) minus required Voltage (1.5 Volts) divided by current (in Milliamps). In this example, Voltage would be 10.5 that needs to be dropped by the resistor. If you have a 100 Milliamp bulb, the required resistance would be 10.5 divided by .100, or 105 Ohms.
105 Ohms is not a standard resistor value, so you should use the next higher available value, which would be 110 Ohms. You would also need a 1 Watt resistor, which is going to be significantly large.
The main point is, you must know the current draw of the bulbs in order to know what value of resistor is required.
What are you using the bulbs for?
Personally, I would never install an incandescent bulb in a locomotive or use them to light passenger cars. Tomar caboose lanterns are incandescents, so I’ll make an exception there. A big advantage of LEDs is that, once wired properly, they basically last forever and you don’t have to replace them. Also, LEDs run very cool, so you won’t melt your locomotive shell.
On the other hand, I prefer incandescents for structure lighting and streetlights. You can wire multiple 1.5 volt bulbs in series, rather than in parallel, to reduce the voltage going to each. If you take ten 1.5 volt bulbs in series and connect them to a 12-volt source, each bulb will get 1.2 volts and you will not need any resistors.
I use the same 470 ohm resistor for either an LED or Miniatronic 1.5 volt bulb with my NCE Power Cab. Me, I prefer using the bulbs because I like the look of the light and its ‘warmth’. I feel that LED’s are much too bright. I do like my bulbs a little dim.
To be honest, I don’t run my stuff enough to burn one out, but if I do, it will give me the opportunity to clean and lube the loco while it’s apart.
I’m not going to knock calculating resistance. It’s important if you’re uncertain of the right resistor for the application. But I have a whole fleet of Genesis F units that received 1/2 watt-rated resistors to run those 1.5 volt bulbs that Athearn uses when they were converted to DCC. Never had a problem with one overheating. Perhaps the passage through the decoder adds resistance or otherwise has something to do with it?
I was mainly asking in regards to structures & streetlights. I’m currently using (no pun intended!) the Low Voltage Lighting Circuit found on Spookshow’s website; and while effective for my blocks of lights, in parallel, the question arose of dimming individual blocks & the unlikelyhood of finding a small enough potentiometer for each block. I remembered then that I had been given a number of old Tyco transformers, & thought that I could power each block independently, & use the ‘speed control’ on each transformer as the dimmer…
I do like the idea of wiring the lights in the blocks in series…
Spookshow’s circuit is actually the one I use, but all LED. I keep it down low, around 3.2V IIRC, and power the LEDs straight off the juice. I also build them in series and apply additional resistance to get lots of variability in the resulting light output. People often think that this can’t be done with LEDs, but it ends up being even simpler IMO.
More on all this at The Night Scene: http://cs.trains.com/mrr/f/88/t/213765.aspx
Which is not to say it doesn’t work with bulbs, too. It will, just more heat involved, because of the profligate, promicuous use of energy with bulbs…[;)][swg][:-^]
Well, Mike, it’s a drafty old apartment, and a cold winter, so every little bit helps…for now I’m keeping my output way down around 1.34V DC, just enough to light everything, until I get all 41 installed…but the genesis of the question came from the fact that I’ve got a fibre optic, side-lit inspection pit, inside my 5 track maintenance shed, & was concerned that the overhead lights inside the shed might overpower the inspection pit, spoiling the effect. Hence the search for individual dimmers for each block.
Spookshow’s lighting control raises another question; it’s designed to use a 9V 2A source. Would dimming the input voltage have a comensurate effect on the overall output?
Been awhile since I installed them, but it should not be a problem to try it. I’m pretty sure I used several random volatge ones, but all at least 6 volts. So long as you’re able to still adjust the LM317 to the required voltage, you should be good. I usually check my newly installed power supplies for execessive heat in the first hour or so just to be sure everything is copacetic.